1. Field of the Invention
The present invention relates to control over pull-up/pull-down on an input terminal of a semiconductor integrated circuit.
2. Description of the Background Art
In an open state of a semiconductor integrated circuit in which nothing is connected to an input terminal of the semiconductor integrated circuit, more specifically, a state in which a specific voltage or current is not applied to the input terminal of the semiconductor integrated circuit, an input voltage is indeterminate, to cause malfunction in outputting. A typical solution for avoiding such situation is to provide a control circuit for performing pull-up or pull-down on the input terminal of the semiconductor integrated circuit.
Which of pull-up or pull-down is performed depends chiefly on logical relationship between an input signal to be pulled up or down and an output signal associated with the input signal. Either pull-up or pull-down is selected to be performed in accordance with logical relationship between an input signal and an output signal associated with the input signal.
In the conventional art, as respective logics of an input signal and an output signal are uniquely determined, which of pull-up or pull-down should be performed on an input terminal is uniquely determined.
For this reason, when it becomes necessary to change logical relationship between an input signal and an output signal, or to switch between pull-up and pull-down in a semiconductor integrated circuit, an additional design for achieving new logical relationship between input and output signals, or for achieving either pull-up or pull-down, is required. This increases manufacturing cost as well as labor for managing products, disadvantageously.
It is an object of the present invention to obtain internal or external control over selection between a pull-up resistor and a pull-down resistor when no input signal is supplied, and change of logics of input and output signals in a semiconductor integrated circuit device.
According to the present invention, a semiconductor integrated circuit device includes a power supply line, an input terminal, an output terminal, a control terminal, a ground terminal, a first MOS transistor, a second MOS transistor, an input/output logic changing circuit and a control signal line.
The first MOS transistor includes a first main electrode connected to the power supply line, a second main electrode connected to the input terminal, and a first gate electrode. The first MOS transistor flows a pull-up current from the first main electrode to the second main electrode in a first on-state where the first MOS transistor is in an on-state when no input signal is input to the input terminal.
The second MOS transistor includes a third main electrode connected to the input terminal, a fourth main electrode connected to the ground terminal, and a second gate electrode. The second MOS transistor flows a pull-down current from the third main electrode to the fourth main electrode in a second on-state where the second MOS transistor is in an on-state when no input signal is input to the input terminal.
The input/output logic changing circuit includes a first terminal connected to the input terminal, a second terminal connected to the output terminal, and a third terminal.
The control signal line includes one end connected to the control terminal and the other end connected to the first gate electrode, the second gate electrode and the third terminal. The control signal line transmits a control signal thereon to the other end.
When the control signal has a first level, the first MOS transistor and the second MOS transistor are placed in the first on-state and a second off-state where the second MOS transistor is in an off-state, respectively. When the control signal has a second level, the first MOS transistor and the second MOS transistor are placed in a first off-state where the first MOS transistor is in an off-state and the second on-state, respectively.
When an input signal is input to the input terminal, the input/output logic changing circuit determines a logic level of an output signal based on the input signal and the control signal to thereby output the output signal to the output terminal.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.